1. Field of the Disclosure
The present disclosure generally relates to paints, optical and specialty coating sensor materials that utilize the diffraction properties of photonic crystals.
2. Brief Description of Related Art
Color shifting paints and coatings have been proposed for numerous applications, including but not limited to, such applications as automobile paints to anti-counterfeiting inks for security documents and currency. These color shifting paints and coatings contain pigments that change their color upon variation of the angle of incident light, or when the angle of view is shifted. Several color shifting technologies has been proposed in the related art and are briefly described below.
For example, U.S. Pat. Nos. 3,087,828 and 3,087,829 disclose pearlescent pigments (interference pigments) that are comprised of synthetic or nonsynthetic mica particulates which are coated with a metal oxide layer(s). As a result of reflection and refraction of light and the angle of which the surface is observed, the particulates will show a pearl-like diffraction. The color of the light reflected by the particulates also depends on the thickness of the metal oxide coating on the particulates. The most common of the metal oxides used for coating the particulates are titanium and iron oxide which can be used in a single coat or in a mixed coating of the mica particulate. However, this color shifting technology yields low diffraction efficiency and also results in a relatively high cost.
United States Patent Application Publication Number 2003/0125416 discloses another color shifting technology, more specifically color effect composition pigments that provide a goniochromatic finish. These types of pigments are comprised of an ordered periodic array of monodisperse particles of defined thickness and surface area which are embedded in a polymer matrix. These types of pigments are similar to that of pearlescent pigments in that particulates of the ordered periodic structure are fixed into the matrix solution after their creation and incorporation into the matrix. A result of this is that a two step process is required. The particulate pigment flakes must be made first and then redispersed into a suitable matrix which will allow a film forming coating. This two steps process yields low diffraction efficiency due to relatively low crystalline array solid content and requires longer processing time and cost.
Additionally, diffraction patterns and holographs are also employed in color shifting applications due to their visual effects. This visual effect occurs when ambient light is diffracted from a diffraction grating. Diffraction gratings are repetitive structures made of lines or grooves in a material to form a periodic refractive index structure. Diffraction grating technology is employed in the formation of two-dimensional holographic patterns (for example United States Patent Application Publication Number 2004/094850 A1).
However, it is desirable to devise a non-expensive film forming color shifting paint and coating with a high diffraction efficiency. It is desirable to devise a non-expensive film forming paint that efficiently Bragg diffracts light in both wet and dry states in the visible and NIR spectral regions.
It has been recognized that crystalline colloidal arrays (“CCAs”) diffract visible light according to Bragg's Law. CCAs are highly-ordered, three-dimensional arrays of monodisperse highly-charged particles that self assemble into a body-centered cubic (BCC) or face-centered cubic (FCC) lattice. CCAs Bragg-diffract light at wavelengths determined by the incident angle, the lattice spacing and the refractive index of the array. The lattice spacing of the array can be tuned, either by changing the particle size or the particle concentration, so that CCAs can efficiently diffract light in the visible and NIR spectral regions. Highly monodispersed polymer particles or spheres prepared by emulsion polymerization have been mainly utilized for this purpose due to their high monodispersity and high surface charge. Since the ordering of the CCAs depend on the electrostatic repulsion between the particles, the lattice will disorder in the presence of ionic impurities. Therefore, it is desirable to devise highly ordered CCAs of highly charged particles that maintain their diffraction in the presence of ionic impurities such as may be present in paint formulations.
Paints and coating formulations contain a film-forming material (resin). These film forming materials may be organic or inorganic and may form a film after the drying of the paint. The film forming material may be transparent or may contain pigments. The resin may hold the pigment particles close enough together and may attach them to the surface that they have been applied on. Some commonly applied film forming mechanisms in paints are: reaction with oxygen in air (oxidation), solvent evaporation and coalescence above the minimum film forming temperature (MFFT), and chemical cross-linking (polymerization).
It is desirable to devise a film forming paint and coatings that Bragg diffract light in the visible and NIR spectral regions. This, and other developments are a result of the present disclosure.